People are most likely to attribute someone's behavior to internal causes when that person's behavior is:

A) the same thing the observer would have done.
B) highly variable from one situation to another.
C) not a free choice.
D) unusual or surprising.

People are most likely to attribute someone's behavior to internal causes when the behavior is unusual or surprising, as it does not fit the expected norms or patterns, leading observers to infer that the behavior must be due to the individual's personal characteristics or dispositions.